The present invention is related to integrated circuits. More specifically, the present invention can be applied to devices used controlling power supply. According to various embodiments, the present invention provides various power control schemes to improve system stability and performance. Merely by way of example, the present invention can be used in a power system that is selectively operable in various power modes based on operating conditions, such as the output power level, power consumption, etc. It is to be appreciated that the present invention has a broad range of applications.
The pulse width modulation (PWM) technique is widely used in power supply systems. More specifically, the PWM technique has been adopted in many regulated switch mode power converter designs. Among other thing, a benefit of the regulated switch mode power converters is their higher efficiency comparing to linear regulator. As energy saving requirement and efficiency regulations becoming more stringent, regulated switch mode power converters are becoming increasingly more popular. In comparison, linear regulators are becoming less popular and may be out of market for its poor power efficiency and power saving capabilities. In the near future, switching power converters are likely to become the main topology for power supply system.
In the switching power conversion systems, power switches are often implemented using power MOSFET, Power bipolar transistor, IGBT or other type of transistor as the switching elements. The amount of energy transferring is regulated by power switching on and off time. For example, the on and/or off time of the PFM signals are controlled in accordance with the output load. In a specific example, the output voltage and/or current is regulated by sensing the output voltage and/or current and applying the corresponding control signals to the power switches.
FIG. 1 is a simplified diagram illustrating a conventional flyback switching power conversion system. The system 100 includes a PWM controller 101 is used to control and drive a power MOSFET 102, which turns on and off to control the power delivered to the load in the secondary side. The output voltage and current are sensed in the secondary side and compared with the desired output voltage and current reference. The error signal is amplified and feedback to the controller in primary side via a photo-coupler. Usually, a TL431 (e.g., a component for providing isolated feedback) or similar type of ICs and PC817 (e.g., a photo coupler) or similar type ICs are used in the secondary side for the isolation feedback.
FIG. 2 is a simplified diagram illustrating an alternative conventional flyback switching power conversion system. As shown in FIG. 2, a system 200 has a primary side and a secondary side. As shown, sensing, amplification, and the signal transferring (e.g., accomplished by the photo coupler in system 100) are omitted, which largely reduces the manufacturing costs of the system 200. In the primary side controlled switched power conversion system, the output voltage is imaged to the auxiliary winding (AUX). As a result, the output voltage can be sensed by monitoring the voltage at auxiliary winding (AUX). In various application, the output voltage is regulated by comparing the voltage at AUX with the desired voltage reference. More specifically, the output voltage signal is mapped to the signal at node 201. The regulation of voltage at the node 201 translates into the regulation of the output voltage.
To further illustrate, the primary side regulation, the relationship of VFB and Vout is expressed as:
                                          V            FB                    =                                                    n                ·                                  R                  2                                                                              R                  1                                +                                  R                  2                                                      ·                          V              out                                      ,                            (        1        )            
Where n is the ratio of auxiliary winding to secondary winding.
Setting
      k    =                            R          1                +                  R          2                            n        ·                  R          2                      ,Vout is expressed as the following:Vout=k·VFB  (2)
Both the system 100 and system 200 as illustrated above are useful for certain applications. Unfortunately, these conventional system are often inadequate.
Therefore, improved systems and methods for providing power supply are desired.